Conventional piston rings, including compression rings and side rails of combined type oil rings, of gasoline and diesel engines, are made of martensite-based stainless steel with the surface thereof applied with nitriding treatment, to improve abrasion resistance. As for conventional first compression rings of large diesel engines, rings made of cast iron have been used with the external circumference thereof applied with chromium plating. When the EGR (exhaust gas recycling) system is employed in accordance with the exhaust gas regulations, however, the cast iron applied with chromium plating, comes to lack in sufficient abrasion resistance, whereby the first compression rings are required to improve the abrasion resistance thereof by having the rings made of martensite-based stainless steel, with the external circumference thereof applied with nitriding treatment.
As for second compression rings of large diesel engines, the operational environment in cylinders of the engines, is not so severe as the first compression rings. Nevertheless, a problem of insufficient abrasion resistance occurs in case of conventional rings using cast iron applied with chromium plating, when the EGR system is employed. Besides, when implementing axially thinner second compression rings to improve the engine fuel consumption, the cast iron rings come to lack in sufficient strength. The second compression rings are, therefore, also desired to be made of steel.
Compression rings made of steel are disclosed in Japanese Patent Publications No. 58-25863 (1983) and No. 61-54103 (1986), wherein the first and second compression rings are both provided to be made of an identical steel material.
In case of manufacturing piston rings, including compression rings and side rails of combined type oil rings, for use in internal combustion engines, including gasoline and diesel engines, made of steel material, a problem exists in that the bending work during the ring forming process, is not easy, and occasionally causes a broken ring.
As for compression rings used in large diesel engines, the compression rings, particularly, first compression rings, require a significantly high abrasion resistance, the rings are manufactured using relatively expensive high-Cr steel material. Thus, there has been another problem in that the total manufacturing cost of compression rings is caused expensive when the second compression rings are made of the same steel material as that of the first compression rings.
Moreover, there has existed a further problem in that, when the first and second compression rings are made of an identical steel material, and are applied with chromium plating of an identical hardness, then, the difference between the first and second compression rings, in the abutting gap thereof, grows abnormally great after a long time use, thereby resulting in the compression rings having their sealing capability decreased and causing the blow-by gas volume to increase.
Furthermore, an additional problem exists in that, since the second compression rings, used in large diesel engines, have a larger cross-sectional area than those used in in gasoline engines, the rings exhibit poor cold workability during the ring forming process.